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Time travel is theoretically possible, calculations show – but can we change the past?

Doc Brown and Marty McFly in “Back to the Future”. – Copyright: Universal Pictures

Imagine stepping into a time machine, pressing a button, and traveling back to 2019, before the coronavirus made the leap from animals to humans. What if you could find patient zero and isolate him? In theory, the Covid-19 pandemic wouldn’t exist then, right? Not quite, otherwise you wouldn’t have decided to travel back in time in the first place.

For decades, physicists have studied and debated versions of this paradox: If we could go back in time and change the past, what would happen in the future? A 2020 study provides one possible answer: nothing. “Events adjust around anything that might cause a paradox, so the paradox doesn’t happen,” Germain Tobar, the study’s author, told the journal IFLScience.

Tobar’s work, published in September 2020 in the journal Classical and Quantum Gravity, suggests that, according to the rules of theoretical physics, whatever one tried to change in the past would be corrected by subsequent events. Put simply, it is theoretically possible to go back in time, but history cannot be changed.

The grandfather paradox

Physicists have considered time travel to be theoretically possible since Albert Einstein developed his theory of relativity. Einstein’s calculations suggest that it is possible for an object in our universe to travel in a circular direction through space and time, eventually ending up at a point in its journey where it has been before. This path is called a closed time-like curve.

Still, physicists continue to grapple with scenarios, like the above example of the coronavirus, where time travelers alter events that have already happened. The most famous example is the so-called grandfather paradox: Suppose a time traveler goes back in time and kills a younger version of his grandfather. The grandfather would then have no children, wiping out the time traveler’s parents as well as the time traveler himself. But then who would kill the grandfather?

A twist on this paradox appears in the movie Back to the Future. Marty McFly almost prevents his parents from meeting in the past – possibly causing him to disappear himself in the process.

The billiard ball model

To resolve the paradox, Tobar and his supervisor, Dr. Fabio Costa’s “Billiard Ball Model” which envisions cause and effect as a series of colliding billiard balls and a circular pool table as a closed time curve.

Imagine a row of billiard balls spread out on the circular table. If you nudge a ball from position X, it will bang around on the table and bounce on others, which will then fall into a specific pattern.

The researchers calculated what would happen if you messed up the sphere’s path at a certain point in its journey. Interactions with other spheres can correct its path, allowing it to return to the same position and speed it would have reached if no intervention had been made. “Regardless of the choice, the ball will roll to the same place,” said Dr. Yasunori Nomura, a theoretical physicist at UC Berkeley, to Business Insider.

We cannot change the past

In other words, Tobar’s model says that while you can travel back in time, you can’t change the course of events enough to change the future, Nomura says. Applied to the grandfather paradox, this would mean that there would always be something in the way of trying to kill the grandfather. Or at least the grandmother would be pregnant with, for example, your mother by the time he dies.

Back to the example of the corona virus. Let’s say you travel back to 2019 and intervene in the life of patient zero. According to Tobar’s considerations, the pandemic would still take place somehow.

“You could try to prevent patient zero from becoming infected, but you would catch the virus and become patient zero, or someone else would,” Tobar said in a press release from Australia’s University of Queensland, at Tobar has graduated. Nomura adds that while the model is too simple to capture the full range of cause and effect in our universe, it is a good starting point for future physicists.

This article was translated from English by Zoe Brunner. You can find the original here.




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